A Phase Locked Loop (“PLL”) circuit is an electronic control system that may be used, in part or in whole, to generate or maintain one signal that is locked onto the phase and frequency of another signal. As an example, a PLL may be used as a clock data recovery (“CDR”) circuit to synchronize a clock signal with an input data signal. While particular embodiments of a PLL may vary with respect to their component parts, an example PLL may include a frequency/phase detector, a charge pump, a loop filter, and a variable-frequency signal generator (e.g., a voltage controlled oscillator (“VCO”)). In an example situation, a PLL may receive a reference signal from an outside source (e.g., an input signal from a remote data source) and may compare the phase of the reference signal with a locally-generated variable-frequency signal. The locally-generated variable-frequency signal may originate, for example, from a variable-frequency signal generator over which the PLL has control. As part of its comparison, the phase detector may determine a phase difference between the reference signal and the locally-generated variable-frequency signal and may use the phase difference to generate an error correction signal. In particular instances, the error correction signal may be input into a charge pump that constitutes part of the PLL. The charge pump may generate a current in response to the error correction signal. In turn, the current generated by the charge pump may be taken to a loop filter and may dictate the output voltage of the loop filter. The output voltage from the loop filter is fed back to the variable-frequency signal generator to control the frequency of the signal generated by the signal generator. By controlling the frequency of the signal generator using the charge pump output, the PLL may adjust the frequency of the signal generator to match the frequency of the reference signal.
Because the frequency of the variable-frequency signal generator may ultimately depend on the current generated by the charge pump, the “linear range of operation” of the charge pump is one metric used to gauge the effectiveness of charge pumps. The liner range of operation may be referred to as the range of voltages for which the charge pump can output a constant current (e.g., for which a specified output resistance holds).